The annual cost of physical frailty in the United States is enormous, $80 billion, yet the cause of muscle atrophy and weakness is unknown. The working hypothesis of this Program Project is that the atrophy and weakness of skeletal muscles in old animals is caused by the imbalance between the generation of reactive oxygen species (ROS), particularly during demanding protocols of aerobic contractions, and in the presence of deficiencies in antioxidant systems. The purpose of the Program Project is to determine the effect of a demanding isometric contraction protocol on the generation of ROS, ROS-induced damage and adaptations of heat shock proteins in skeletal muscles of 8-month old (young/adult) and 28-month-old (old) mice. Comparisons will be made among wild type (WT) mice and mice with mitochondrial or cytosolic antioxidant systems deficiencies due to partial (Sod+/- and Sod1+/-) or complete (Sod2D3-/- and Sod1-/-) knockouts, or mice enhanced by transgenic procedures (Sod2Tg+/-) or by aerobic conditioning. In aerobic organisms, ROS are generated constantly with 85% generated in the mitochondria and the remainder from extra-mitochondrial sources. Particularly for skeletal muscles of young healthy animals with intact or enhanced antioxidant systems, even with the increased ROS generation of a demanding aerobic contraction protocol, the generation of superoxide anions produces an ROS stress and adaptations in HSPs, but no ROS damage. In contrast, for skeletal muscles of old animals, or animals with impaired antioxidant systems, ROS stress has the potential to produce sufficient damage at rest or with contract protocols to cause atrophy, weakness and loss of power. The working hypothesis will be tested rigorously through experiments on four Group Hypotheses tested collaboratively, and specific hypotheses, and specific hypotheses tested on young/adult and old mice by Project #1 on conditions of ROS stress and damage following a demanding aerobic contraction protocol; by Project #2 regarding mitochondrial function of muscles from WT and Sod2 deficient and enhanced mice, and by Project #3 comparing expression patterns of satellite stem cells from different groups. The projects will be assisted by Administrative, Transgenic Animal, and Biochemistry Cores and a Statistical Unit.